The invention relates generally to pulse time of arrival location methods and systems. More particularly, the invention is directed to a system and method for estimation of the time of arrival of a received signal in a wireless communications system, utilizing cyclic carrier de-rotation and amplitude-offset invariant template detection for pulse position estimation to determine the earliest time of arrival of a line-of-sight signal among a plurality of signals which may include the line of sight signal and one or more multipath signals.
The operation of many discrete systems is based on analysis of a signal in order to find the existence and position in time of a particular signal with a predetermined shape. Laser-scanner decoding, mobile ranging, real-time location, and digital signal processing for feature extraction all may be based on deriving knowledge concerning the time of arrival of one or more signals of predetermined characteristic and duration. This type of analysis can be useful in any application where an output or intermediate stage is designed to produce an amplitude-varying signal whose xe2x80x9cpulsesxe2x80x9d correspond to particular events of importance. Determination of the position of these pulses may be accomplished by a variety of methods, including comparison of the amplitude to some set threshold and the novel approach presented in xe2x80x9cSpread Spectrum Indoor Geolocation,xe2x80x9d by B. Peterson et al., published in the Journal of the Institute of Navigation, Vol. 45, No. 2, Summer 1998, incorporated herein in its entirety by reference. (Reference I). In geolocation systems, pulse position detection can be used to determine the time of arrival of a transmitted signal at different receiver locations. Based on the time of arrival of the signal at these receiver locations, triangulation algorithms can be used to estimate the location of the transmitter.
The accuracy of many pulse-position detection systems, however, may become suspect in the presence of multipath impairments (i.e., the combination of random, multiple time-shifted versions of the transmitted signal along with the direct line-of-sight received signal). Because multiple versions of the transmitted signal may be randomly interspersed on the communications channel, it may become considerably more difficult to apply a template detection system to identify the existence and position in time of a particular received signal.
U.S. patent application Ser. No. 10/242,363 provides a method for amplitude-offset invariant template detection for pulse position estimation which resolves many of the difficulties encountered by template detection systems with respect to amplitude variations which may occur in a multipath environment. However, this related application of a method of amplitude-offset invariant template cross correlation, when applied to communications systems for time of arrival estimation, requires that the carriers of the received line-of-sight and multipath signals be perfectly aligned in phase with each other so that the sum of the two transmitted signals would result in a receiver output equivalent to a linear combination of the two individual receiver outputs. The system and method of the present invention takes the templating procedure (and other time of arrival estimation algorithms which rely on this assumption) a step further, applying it to the case of time of arrival estimation in communications systems, where, because of differences in the carrier phases of the various multipath and line-of-sight signal components, important timing information may be corrupted or lost. In these applications, multipath signal components may dominate over line-of-sight components, and the templating procedure cannot be guaranteed to yield the optimal time of arrival estimate due to the random variations in phase of the received signal. Subsequent processing may be needed to extract the true earliest time-of-arrival of the received signal.
Accordingly, a need has arisen for a pulse detection system which can compensate for random phase shifts that may be introduced into a signal transmitted in a multipath environment by de-rotating the carrier phase before applying a time-of-arrival estimation algorithm. In this way, the phase of the received signal may be shifted by some angle to isolate the component in the signal with the least multipath effect, thereby better isolating the line-of-sight signal and improving the time-of-arrival estimate. The present invention accordingly provides an efficient system and method for accurately identifying the earliest time of arrival of a received signal.
In a first embodiment of a system and method according to the present invention for estimating the earliest time of arrival of a received signal, a complex vector having a first phase reference can be formed by combining one or more in-phase and quadrature components of a received signal, computing the time of arrival of the received signal, varying the phase reference of the received signal one or more times to compute additional time of arrival estimates based on derivative complex vectors, and then comparing the time of arrival estimates to determine the true earliest time of arrival of the receives signal.
A next embodiment of a system according to the present invention for estimating the earliest time of arrival of a received signal may comprise a receiver, an analog-to-digital converter, apparatus for combining the in-phase and quadrature components of a received signal into a complex vector, apparatus for shifting the phase of the received signal, apparatus for computing the time of arrival of the received signal, and at least one comparing apparatus for comparing one or more computed times of arrival to determine the true earliest time of arrival of the receives signal.